This invention relates to a method of surface treating thin coatings of metal or semiconductor oxides to improve their adherability.
Many thin coatings are subjected to subsequent processing steps in the manufacture of a finished product. These may include coating with a different material, marking, and lamination. In all such steps the coating must come into intimate contact and form a bond with an adherend. Often, the strength of this bond is important if not critical to satisfactory performance of the product. It has been proposed to improve the bond strength by treating the surface of either the coating or the adherend or by applying an intervening primer layer which will bond well to both the coating and the adherend.
Thin coatings made of oxides such as indium tin oxide (ITO) and tin oxide are useful in a number of products, ranging from electrooptical displays (e.g., electroluminescent or liquid crystal displays) to membrane switch panels to coatings having controlled optical properties in the infrared, visible, or ultraviolet spectral regions or specific combinations of electrical and optical properties. In some products, they may serve as a transparent electrical conductor. In other products, they may be designed to reflect light of a particular wavelength while transmitting light of a different wavelength either alone, or in conjunction with other layers of metals or metal oxides. Thus, depending on the product, either their electrical or optical properties, or both, is important. Also, in many applications these coatings must be in adherent contact with adjacent layers of material. However, these coatings do not always form good bonds with various other coatings and inks, unless they are treated to improve adhesion. Typically, problems occur with polar, water-borne, or hydrophilic (polar) adherends.
Many different kinds of primer layers are available to improve bond strengths. Arkles, in "Tailoring Surfaces with Silanes," Chemtech, Dec. 1977, p. 776, discusses the use of organosilanes for this purpose. Product literature from Du Pont on their Tyzor (TM) organotitanates (Brochure No. E69996-1, 4/87) teaches the use of these materials for adhesion promotion. (Such silanes and titanates are sometimes referred to as coupling agents.) However, the use of a primer layer or coupling agent entails the deposition of an intermediate material between the coating and the adherend, with an uncertain prognosis as to the long-term chemical compatibility of the three elements and an extra interface which must also have good bond strength. Also, the uniform deposition of a primer layer may require special equipment.
Known surface treatments include exposure to corona discharge, as discussed by Stobbe in Paper, Film and Foil Converter, Feb. 1988, p. 72 and in product literature by Enercon Industries Corp. (Menomonee Falls, Wis. It has been proposed to use a glow discharge in Handbook of Thin Film Technology, Maissel and Glang, Eds., pp. 6-41 (McGraw-Hill 1970), particularly in the presence of oxygen, for the purpose of volatizing organic residues by chemical reaction and of surface modification of the thin film.
In Handbook of Adhesive Bonding, Cagle, ed., p. 14-4 (McGraw-Hill 1982), a surface treatment for steel is taught, in which the steel is treated with an aqueous mixture of a strong acid (hydrochloric acid), hydrogen peroxide, and hexamethylene diamine. Apparently the former etches off any oxides on the steel surface, to provide a clean metallic bonding surface. The diamine, a chelating agent, then forms a passivating layer on the metal. The amino groups of the diamine make this technique particularly useful for enhancing adherability to amine-reactive materials such as epoxies and polyurethanes.
Kujawa et al., in U.S. Pat. No. 3,888,702 (1975) discloses another surface treatment using hydrogen peroxide. In this case, nickel is treated with aqueous hydrogen peroxide containing lead ion to enhance its bondability to silicone rubber.
In considering surface treatments, a distinction must be made between a treatment to remove contaminants and a treatment to modify the surface to improve its adherability. Contaminants are usually either particulate or organic residues and can be removed either by mechanical means, solvents, or combinations of both techniques. Sometimes both contaminant removal and surface modification may be required.
ITO and other metal, mixed metal, or semiconductor oxide coatings are typically deposited by vapor deposition techniques, among which sputtering is one of the better known ones. Vapor deposition techniques are considered to be "clean" techniques, that is, they deposit a uniform, uncontaminated coating. We have found that for such coatings, having a cleanly deposited surface is not a sufficient condition for ensuring good adherability and that their adherability is markedly improved by a surface treatment according to our invention. It is often necessary, due to the nature of the substrate or of the end product, for the freshly deposited coating to come into contact with another surface. For instance, in coating a roll of polymer film the coating is wound up upon the reverse side of the film. Similarly it is often necessary to put a protective cover sheet on a coating soon after removing it from the coating apparatus. We have discovered a new method for improving the adherability of thin metal or semiconductor oxide coatings.